1. Field of the Invention
The present invention relates to an oil-in-water emulsion sunscreen cosmetic composition. More specifically, the present invention relates to an oil-in-water emulsion sunscreen cosmetic composition having high protection ability against UVA and excellent emulsion stability, which are achieved by adding a hydrophobized zinc oxide satisfying certain physical conditions as an ultraviolet scattering agent as well as a quaternary ammonium compound having a long-chain aliphatic group as an emulsion stabilizer.
2. Description of the Related Art
An oil-in-water emulsion sunscreen cosmetic having a hydrophobized ultraviolet scattering agent dispersed in an internal phase is useful as a formulation system which can provide not only a fresh sense of use but also high protection against UV rays. As an ultraviolet scattering agent, titanium oxide and zinc oxide are mainly employed. However, whiteness and squeaky feeling of titanium oxide stand out in comparison with zinc oxide. Thus, in order to prepare an oil-in-water emulsion sunscreen cosmetic composition having excellent appearance and a sense of use, it is desirable to use zinc oxide. However, it is difficult to stably disperse zinc oxide in an oil-in-water emulsion composition. In addition, if a large amount of zinc oxide is added, an application surface looks excessively white and powdery, with the result that satisfactory end result and texture tend not to be obtained.
To overcome such problems on dispersibility of zinc oxide, the following methods have so far been proposed. JP-A 2009-084171, the entire contents of which are incorporated herein by reference, proposes a method of enhancing dispersibility of zinc oxide by treating the surface of zinc oxide fine particles with a silicone compound having a specific structure and using volatile organopolysiloxane in combination with a silicone dispersant. JP-A 2012-111726, the entire contents of which are incorporated herein by reference, proposes a method of improving dispersion stability of zinc oxide in an oil-in-water emulsion composition by hydrophobizing zinc oxide fine powder with octyltriethoxysilane and/or dimethylpolysiloxane and using a certain silicone or sugar ester in combination.
The stability of zinc oxide is relatively satisfactory if the particle size of zinc oxide is several nm or less; however, particularly, it is difficult to stably disperse a zinc oxide powder having a particle size larger than this in an oil-in-water emulsion composition. Thus, improvement is still desired.